Rocks to surround pond (Nunnery Wood WR5)

We are trying to create a water feature to contain water from the roof and ultimately contribute to reducing flooding downstream in Bow brook, the river Avon and ultimate the river Severn by diverting gutter water to butts which will feed the pond. And build a swale along the contour line to contain rainwater and plant it to absorb and slow runoff.

Any old surplus rockery stones welcome, happy to remove from insitu if needed. Thank you!